1. Field of the Invention
This invention relates to an exposure apparatus for reproducing a fine pattern of a mask, for example, for manufacturing semiconductor devices onto a photo-sensitive surface of a substrate using holographic techniques.
2. Description of the Prior Art
As to a holographic technique useful for exposure to reproduce a fine pattern of integrated circuits onto a resist layer which is coated on a wafer, K. A. Stetson discloses a report entitled "Holography with Total Internally Reflected Light" in Applied Physics Letters, Vol. 11, No. 7, Oct. 1, 1967, pp.225-226.
Recording and reconstruction of a hologram of the type mentioned are performed in the following manner. First, upon recording operation, a coherent light beam is irradiated upon a mask, and a subject beam transmitted through and diffracted by the mask enters into a recording medium while another coherent reference beam enters into the recording medium from the opposite side to the mask by way of a prism. Then, the reference beam is totally internally reflected from a boundary of the recording medium from the air. Consequently, the incident and totally reflected reference beams and the subject beam interfere with each other within the recording medium to form interference fringes corresponding to the mask pattern in the recording medium and record them as a hologram. Meanwhile, reconstruction of the thus recorded hologram is performed by irradiating a reconstructing beam conjugate with the reference beam into the hologram recording medium. In this instance, if a wafer is disposed at the position of the mask in place of the mask, then a real image (mask pattern) of the hologram is formed on the wafer. For example, N. J. Phillips actually discloses, in U.S. Pat. No. 4,857,425, a process for manufacturing integrated circuits using holographic techniques of the type just described.
Conventionally, silver halide or dichromated gelatin (DCG) is employed as a recording medium for a hologram. Silver halide photo-sensitive materials are advantageous in that the sensitivity is high and the photo-sensitive wavelength band is wide while DCG materials are advantageous in that a high diffraction efficiency is obtained and the scattering is little. However, both materials require development and fixation by liquid after exposure, and in order to perform those processes, it is necessary to remove the recording medium once from the exposure apparatus after exposure.
In recent years, photo-polymeric materials have been and are being developed as the recording medium for a hologram. Among photo-polymeric materials which are utilized as the recording medium for a hologram, those materials of the type wherein interference fringes are formed on the real time basis simultaneously with exposure, that is, those materials which require substantially no development, are considered promising. However, also photo-polymeric materials of the type just described require a process of fixing interference fringes (hologram) by irradiating light such as ultraviolet rays uniformly over the entire area of the material after exposure for formation of the hologram. Further, a post-baking process for increasing the refractive index of the hologram by uniform heating is sometimes required. Those processing steps are performed after the recording medium onto which a hologram has been formed is removed once from the exposure apparatus.
As regards exposure of a mask pattern using holography techniques, it has been reported already that a fine pattern of a line width of, for example, 0.5 .mu.mm or so can be reproduced. To this end, however, it is necessary for a reconstructing beam, which is completely conjugate with a reference beam used for formation of a hologram, to enter into the recording medium to obtain a reconstruction image of the hologram. Accordingly, even if an exposure optical system for a recording operation and a reconstruction optical system for a reconstructing operation are incorporated in the same apparatus, if the recording medium is removed from the apparatus in order to perform developing and fixing processes after a recording operation, then an alignment error occurs when the recording medium is installed into the apparatus again, and consequently, it cannot be avoided that the fidelity of a reconstruction image of a fine pattern is deteriorated. Further, since it must be prevented, in a developing or fixing process which is performed outside the apparatus, for unnecessary light to enter into the recording medium for a long period of time until the process is completed including removal and re-mounting of the recording medium, a substantially dark room process is required. Besides, since much time is required for those processes, the problem is pointed out that the operation efficiency is low and the through-put cannot be enhanced.